The use of drilling fluids in drilling oil and other subterranean wells is well known and numerous drilling fluids have been proposed heretofore. A drilling fluid serves a multiplicity of functions. It must have a high enough viscosity to carry the cuttings to the surface and have a low viscosity under the high shear condition at the drill bit to keep power requirements low.
One of the most important functions of a drilling fluid is to seal off the face of the well bore so that the fluid is not lost to the formation being drilled. Ideally this is accomplished by depositing a thin film of the solids in the fluid over the surface of the borehole without any loss of solids to the formation. In other words the drilling fluid solids bridge over the formation pores rather than permanently plugging the pores. Unfortunately most known drilling fluids are damaging fluids in that in effectively sealing off the borehole they permanently plug the formation to a certain extent in the area surrounding the borehole. Consequently when oil is subsequently obtained from the formation the flow thereof is reduced to the extent of the pluggage which occurred by the drilling fluid solids. An ideal drilling fluid, from the standpoint of formation damage would be one which upon flow through a formation or test core in one direction sealed the core almost immediately and upon flow in the other direction or backflow showed no reduction in the original permeability of the core.
Various additives and components have been used in drilling fluids of the prior art. Polysaccharides have been used as the basic means of viscosity control because of their desirable thixotropic characteristics. These materials are formed by the action of bacteria of the genus Xanthomonas, preferably Xanthomonas campestris, on carbohydrates such as glucose, fructose, sucrose and the like to produce a heteropolysaccharide. The preparation of polysaccharides useful in drilling fluids is well known and is described in the following U.S. Pat. Nos.:
______________________________________ 3,000,790 A. R. Jeanes etal Sept. 19, 1961 3,020,206 J. T. Patton etal Feb. 6, 1962 3,243,000 J. T. Patton etal Mar. 29, 1966 3,319,715 C. F. Parks May 16, 1967 3,699,042 W. C. Browning etal Oct. 17, 1972 ______________________________________
The use of lignosulfonates in drilling fluids as thinners or dispersants is also well known. They can be prepared by procedures described in Reissue U.S. Pat. No. RE 18,268; INDUSTRIAL AND ENGINEERING CHEMISTRY, Volume 31, No. 11, Nov. 1939, pp. 1331-35 and U.S. Pat. No. 3,699,042.
U.S. Pat. No. 3,699,042 described a drilling fluid in which the polysaccharide component is cross-linked with a complex formed by the reaction of a lignosulfonate and chrome sulfate, the chromium content of the complex being 4-10%. Numerous other patents, e.g. U.S. Pat. No. 3,243,000, also relate to the use of chromium to cross-link the polysaccharide. However, we have found that such fluids tend to permanently plug the formation to a significant extent.